What Happens to Stagnant Water in a Vacant Building?

From temporary shutdowns to seasonal occupancy, long periods of reduced water use can lead to a variety of potential hazards when turning the tap back on in a vacated building.

Stagnant water left in unused plumbing is a breeding ground for bacteria and contaminants like lead. Water quality issues can develop within just a few days at individual faucets. Even devices normally used to improve water quality—like filters, heaters, and softeners—can create health risks if not maintained during periods of stagnancy. While there is currently no standard for safely reopening a building, flushing fixtures has been a recommended practice before using inactive plumbing.

Why Does Bacteria Grow in Unused Plumbing?

Stagnant water can spread the growth of Legionella and other biofilm forming bacteria (like Pseudomonas and nontuberculous mycobacteria). Legionella infects people who breathe in droplets or steam from contaminated faucets, causing a pneumonia-like illness.

Legionnaires’ disease, the illness caused by Legionella, kills more people in the U.S. than any other waterborne pathogen.

Plumbing Conditions That Cause Bacteria Growth

How Long Does it Take for Bacteria to Grow in Plumbing?

If present, Legionella can grow significantly within weeks or months of inactivity, depending on factors like water heater temperatures, residual disinfectant concentrations, and preexisting Legionella colonies. Microbes like coliform, pseudomonads and other heterotrophic bacteria are prone to regrowth. 

It’s also important to note that, in addition to pipes, point-of-use faucet filters and water storage units are susceptible to bacteria.

Even Temporary Closure Can Cause Plumbing Corrosion

Stagnant water can also cause harmful metals to dissolve into your plumbing. 

Sitting water releases dissolved oxygen and, in turn, oxidizes metals and promotes the growth of bacteria that scavenge the metal surfaces of pipes for energy. Over time this mix of bacteria, oxidation, and other water quality conditions (like acidity) leads to corrosion, causing metals—like lead—to dissolve into the building’s water system. 

Buildings that have lead, brass, or copper pipes, solder, or fixtures are especially at risk of corrosion.

Lead and copper contamination are linked to health hazards like neurological disorders, heart disease, and digestive distress. Buildings constructed before the 1986 ban on lead plumbing are more likely to contain lead pipes and solder. Fixtures and faucets purchased before 1997 can contain up to 8% lead, and only faucets purchased after 2014 are required to contain no more than 0.25% lead.

Is It Safe To Restart Plumbing in Vacant Buildings?

As noted by Purdue University Center for Plumbing Safety, there is no government or industry standard for how to restart plumbing in large buildings that have been vacant for long periods of time.

While guidance exists for cases like seasonal hotels, they don’t explicitly deal with how to handle long bouts of stagnation in low-occupancy buildings.

In a study funded by the National Science Foundation, Purdue recommends flushing fixtures at least weekly.

Flushing calls for running water from the point-of-entry throughout the entire building’s water system, replacing all the old water. 

The study recommends, at minimum, that the water used for flushing should have:

Shock disinfection should also be considered before reopening if there is a likelihood for bacterial contamination or if the occupants of the building are susceptible to illnesses like Legionnaires’ disease.

How to Safely Flush A Building

Because of the wide variety of building and plumbing systems, it’s difficult to provide general instructions on best practices for the length and timing of flushing routines. The effectiveness of flushing can depend on factors like the age and condition of plumbing and the water’s corrosiveness and chemical content. 

The EPA has provided an evaluation checklist to help inform building owners of factors to consider for creating a water maintenance plan.

However, it is critical for owners to contact their local public health officials to check or receive clarification on safety recommendations for specific building types.

The CDC also provides flushing tips for buildings that include:

Should Water Be Tested After a Long Absence?

Flushing is recommended for any period of low water usage in a building, even after leaving your home for a week’s vacation. 

However, testing is the only way to be sure that a building’s water is safe again. 

Whenever returning to a building after extended time away, consider running a basic water test for Essential metals and minerals in the water to be sure your water is safe for consumption. Tests for Legionella and other bacteria are also worth considering before returning to service to ensure flushing or other disinfection techniques have removed any microbial hazards.

Conclusion

Any time away from a building that leaves your plumbing inactive is an opportunity for contaminants to spread. Flushing is a recommended safety precaution for making sure pipes stay safe.

For homeowners and smaller buildings, there is guidance available for clearing out old water from plumbing after disinfection, extended periods of vacancy, or when water service is resumed.

For larger buildings, there are currently no industry standards, but researchers are working with public officials to help owners come up with water maintenance plans.